You guys would do a law school proud. Much of the discussion here revolves around the actual policy considerations regarding privileges.
However, it might help if I clarified a few things. For one, there is is only one privilege of a constitutional nature, and that's privilege against self-incrimination. All other privileges are rules of evidence. That is, they determine what can or cannot be used in a trial.
Another very important consideration is who actually holds a given privilege. You really can't fully analyze the issue without understanding this concept. See, only one person holds the privilege. When I communicate with my pastor, the "priest-penitent" privilege belongs to me, not the pastor. That means my pastor doesn't have the power to waive the privilege on my behalf. That's why a pastor or priest cannot be forced to reveal information.
On the other hand, I can waive the privilege and allow my pastor to reveal information.
The easiest way to describe the distinction is the spousal privilege, because there are actually two spousal privileges. There is a spousal testimony privilege and a spousal communication privilege.
Imagine a bride and groom at their wedding. The groom leans over before the vows and tells his bride, "I killed Tony." Assuming she goes through with the ceremony, (Maybe she hated Tony), she couldn't be compelled to testify against her husband against HER will, because SHE holds the privilege not to testify. She could freely take the stand if she wanted to, but the state couldn't force her to.
On the other hand, if the groom waited until after the ceremony and they were married when he told her about killing Tony, then she couldn't testify against her husband at all -- even if she later divorced him, because HE holds the privilege as it relates to spousal communications.
I hope that made sense. The reason it's important is that journalists are the ones holding the reporter's privilege. They don't need the permission of their source. (Arguably, there is a separate privilege in whistleblower cases, but that's another issue.)
So, attorney-client privilege doesn't make a fair analogy, because the client holds the privilege. When, as here, the reporter holds the privilege and, as a result, the right to waive that privilege, there just isn't a compelling reason to prevent forced disclosure to determine if a crime has been committed.